drm/radeon: switch to using late_initcall

This fixes an issue where we get inited before fbcon when built-in.

Ideally this should work as a non late_initcall, but this fixes it for now.
We also don't suggest people build this in (at least distro maintainers).

Reported-by: Ryan Hope <rmh3093@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>